What commenting platform should I use on my site?

I am building a site and I want it to have engaged readers. I see that a lot of bloggers use disqus and other commenting platforms instead of wordpress default commenting form. Why do they do that? Won't that get them less comments because people need to register to these platforms and set up their accounts to leave a comment?

What do you suggest me to use? WP original form, disqus or some other platform?

Disqus is quit popular and easy for new users to register, don't worry blogger use it simply because it's the most advanced commenting system with tons of features. for me i use the default form with akismet plugin for spam filtering.

I am open to other opinions also!

Well as of today on of the most popular and best one is disqus. And i recommend it. Let me explain why,
1) I said that its popular so any problems you get or any issues you face might have been already faced by someone so you get faster and better answers and your problem gets solved.
2) Next it loads from their side which means even if you have a bad hosting the load times of your site doesn't get really poor because it loads from their server. So even if you have a lot of comments the load times are pretty same, thus not slowing down your site.
3) You asked why users prefer it over the default wordpress system. One it reduces a lot of spam yes seriously, believe me once you get a good site spam is really gonna hit you! And instead of making it tough for the users by signing in , it gives them and you a closer chance to interact(you surely know they ain't bots). And logging in isn't that tough too!

Well as always there are always alternatives. Comment luv pro is one of them and of course the wordpress default comment system with some gunny plugins can also do wonders!

They use disqus because it allows networking, the comments end up being seen by more people. But it's also a privacy concern because you're giving away your visitor's ip, disqus id which gets loaded automatically, etc so you may choose to preserve your visitor's data and use the WP form instead which is loaded from your own server and not some public service. Also, you get to keep the WP comments on your own database, while disqus comments belong to disqus and you can't control anything about it.